The Ace II X (I believe that's the one you're referring to) is the bottom of the barrel Samsung Android. It has a pretty weak processor and 4" display, 768 MB of RAM, and runs 4.0... but it's cheap.
The S II is a classic - 1.2GHz dual-core Exynos processor, 1GB of RAM, 4.3" Super Amoled display, pretty nice camera even to this day.
The S II LTE, originally a Rogers exclusive phone, is similar to the S II X sold by Telus/Koodo except it has LTE and the S II X doesn't. Specs are awfully similar and both got updated to 4.1.
The S III is slowly becoming the new S II in terms of classic status yet it is currently the most popular phone in North America. When it came out, it was one of the largest smartphone screens available (excluding stuff like the Note, a phablet), added in a 720p resolution, newer processor and 2GB of RAM coupled with a sweet camera and an overall big design change. Currently on Android 4.3 and Samsung confirmed it will eventually get 4.4.
The S4 is the newest device but it is approaching 1 year into its life cycle already. It's basically a bump up in specs in every category from the S3 (screen, camera, processor, battery), but it isn't as groundbreaking nor successful as the S3 was. Not a lot of people have upgraded from the S III yet because it will basically do nearly everything you need it do and then some.
The S5 is going to be announced soon (probably on the 24th at MWC) and there's a bunch of leaks on the internet about it already.
Telus only sells the S II X (long discontinued at this point), the S III, and S4 and will more than likely carry the S5 when it comes out.
If you want anything except for the Ace II X, S3 and S4, then you'll have to flock to Kijiji ,Craigslist, etc. for someone selling those models because Telus didn't offer them or has stopped carrying them. You could also look into something like the HTC One or Xperia Z1 because in my opinion, both have better build quality and a nicer looking interface than Samsung's Touchwiz.